1. Field of the Invention
The invention relates to an antenna device and a communication terminal and, more particularly, to a single-feeder antenna device with multiband capability and a communication terminal equipped with the antenna device.
2. Description of the Related Art
In recent years, some cellular phones use GSM (Global System for Mobile Communications) as one of wireless communication systems. The available frequency band in the GSM is, for example, 850 MHz band, 900 MHz band, 1800 MHz band, 1900 MHz band, or the like. In addition, other than the GSM, a UMTS (Universal Mobile Telecommunication System) is employed as a wireless communication system, and the available frequency band of the UMTS is 2 GHz band.
In addition, in an existing art, a wireless communication terminal, such as a cellular phone terminal, capable of handling the above described wireless communication systems has been developed. Such a wireless communication terminal is able to handle a plurality of available frequency bands. In addition, various structures of an antenna device component of such a wireless communication terminal are suggested in order to be able to handle a plurality of frequency bands. Examples of the structures are shown in FIG. 33 to FIG. 35.
Antenna device components shown in FIG. 33 to FIG. 35 are single-feeder antenna device components. In addition, the antenna device components shown in FIG. 33 to FIG. 35 are able to handle 850 MHz band or 900 MHz band in the GSM in a low-frequency band, and are able to handle 1800 MHz band and 1900 MHz band in the GSM and 2 GHz band in the UMTS in a high-frequency band.
The antenna device component 110 shown in FIG. 33 is an antenna device component with a short-circuit parasitic element (see, for example, Translation of PCT Application No. 2006-527949). As shown in FIG. 33, an antenna element 2 of the antenna device component 110 includes a low-frequency band antenna conductor 72 and two high-frequency band antenna conductors 73 and 74. The high-frequency band antenna conductor 74 is formed along the outer side of the other high-frequency band antenna conductor 73, and is not electrically connected to the high-frequency band antenna conductor 73. The antenna device component 110 uses capacitive coupling of the high-frequency band antenna conductor 73 with the other high-frequency band antenna conductor 74 to enable handling a plurality of high-frequency band modes. Note that where the wavelength of a signal in each available frequency band is λ, the path length of each conductor is adjusted to λ/4.
In addition, the antenna device component 111 shown in FIG. 34 is a GF slot-type (type in which a slot is present between a ground portion (Ground) and a feed (Feed) connecting portion) antenna device component. As shown in FIG. 34, an antenna element 2 of the antenna device component 111 includes a low-frequency band antenna conductor 92 and two high-frequency band antenna conductors 93 and 94. In the antenna device 111, these antenna conductors are electrically connected to each other. Then, in the antenna device component 111, the path length of each antenna conductor is varied to handle a plurality of frequencies.
The antenna device component 81 shown in FIG. 35 is a bifurcated element-type antenna device component that performs matching by a parallel resonant circuit 39. As shown in FIG. 35, an antenna element 2 of the antenna device component 81 includes two antenna conductors 35 and 36 and the parallel resonant circuit 39 in which an inductor 37 and a capacitor 38 are connected in parallel. One of the terminals of the parallel resonant circuit 39 is connected to a feed line 11 that connects a feeding point 3 with the antenna conductors 35 and 36, and the other terminal is grounded by a short-circuit line 10.
In the antenna device component 81 shown in FIG. 35, the parallel resonant circuit 39 formed of the inductor 37 and the capacitor 38 is provided to handle a plurality of high-frequency band modes. Specifically, the parallel resonant circuit 39 is designed so that only the inductor 37 of the parallel resonant circuit 39 substantially functions in the high-frequency band mode having a frequency of the lower one. In addition, the parallel resonant circuit 39 is designed so that only the capacitor 38 of the parallel resonant circuit 39 substantially functions in the high-frequency band mode having a frequency of the higher one.
The frequency characteristics of the antenna device components shown in FIG. 33 to FIG. 35 each include a low-frequency band and a high-frequency band. The high-frequency band is formed of three modes, that is, 1800 MHz, 1900 MHz and 2 GHz, so the high-frequency band has a wide-band characteristic. On the other hand, the low-frequency band is formed of a single mode, that is, 850 MHz (or 900 MHz), so the low-frequency band has a narrow-band characteristic.
In addition, in an existing art, it has been suggested that various antenna device components are also able to handle a plurality of frequency bands in a low-frequency band (see, for example, Translation of PCT Application No. 2005-521315, Domestic Re-publication of PCT Application 2004-047223 and “A Brief Survey of Internal antennas in GSM phone 1998 to 2005” (Corbett Roewll, Hong Kong)).
Translation of PCT Application No. 2005-521315 suggests a dielectric-resonator antenna device component. The antenna device component uses a high-dielectric material to have two-resonance characteristics in a low-frequency band, thus obtaining a wide-band characteristic.
Domestic Re-publication of PCT Application No. 2004-047223 suggests an antenna device component called a tunable antenna. The antenna device component includes a frequency band change-over switch. With the change-over switch, the antenna device component handles two modes in a low-frequency band.
In addition, “A Brief Survey of Internal antennas in GSM phone 1998 to 2005” (Corbett Roewll, Hong Kong) suggests a stacked antenna device component. The antenna device component bonds two antenna conductors to have a double-layer structure, thus obtaining a wide-band characteristic in a low-frequency band.